The differences of RAM DDR1, DDR2, DDR3

Speed : DDR3 > DDR2 > DDR
DDR2: 100MHZ - 133MHZ  1600MB/s - 2133MB/s
DDR3 : 133MHZ 2666MB/s

Power consuming : DDR > DDR2 > DDR3
DDR1 : 2.5 V
DDR2 : 1.8 V
DDR3 : 1.5 V

Price : DDR > DDR2 > DDR3
because of its availability.

Size : the 3 RAMs have the same size but different notch position, take a look at the picture.